1. Field of the Invention
The present invention relates to a surface acoustic wave filter having filtering characteristics and, in particular, a balance-to-unbalance conversion function, and to a communication apparatus including the surface acoustic wave filter.
2. Description of the Related Art
Recently, there has been significant technological progress in reducing the size and weight of cellular telephones. In order to achieve these advantages, in addition to reducing the number and size of components, multi-function components are being increasingly developed. In the state of the art, recent research has been actively conducted regarding surface acoustic wave (SAW) filters having a balance-to-unbalance conversion function, i.e., a so-called xe2x80x9cbalunxe2x80x9d function, which are used in the RF (radio frequency) stage of cellular telephones. Such SAW filters are used mostly for GSM (global system for mobile communications) compliant devices.
If a balanced line such as a twin lead feeder is directly coupled to an unbalanced line such as a coaxial cable, an unbalanced current flows, thereby, undesirably, causing the feeder itself to operate as an antenna. A circuit called a balun is thus required for preventing the unbalanced current in order to match the balanced line to the unbalanced line.
Many patent applications pertaining to SAW filters having a balance-to-unbalance conversion function have been filed. A well-known SAW filter having a balance-to-unbalance conversion function and having an input impedance and output impedance which are substantially equal to each other is illustrated in FIG. 19.
In the SAW filter shown in FIG. 19, a comb-like electrode (also called an interdigital transducer, hereinafter referred to as an xe2x80x9cIDTxe2x80x9d) 101 is provided on a piezoelectric substrate 100. IDTs 102 and 103 are positioned on the left and right sides of the IDT 101, respectively, in the surface acoustic wave (SAW) propagation direction.
Reflectors 104 and 105 for reflecting a surface acoustic wave in order to improve the conversion efficiency are further arranged in the SAW filter in such a manner that the reflectors sandwich the IDTs 101, 102, and 103 therebetween from the left and right. The SAW filter further includes balanced signal terminals 106 and 107, and an unbalanced signal terminal 108.
The SAW filter of this type is called a three-IDT longitudinally coupled resonator SAW filter having a balance-to-unbalance conversion function by performing conversion between the IDTs 101, 102, and 103 using a surface acoustic wave.
In a SAW filter having a balance-to-unbalance conversion function, it is required that the transmission characteristics in the pass band between an unbalanced signal terminal and each balanced signal terminal have equal amplitude characteristics and 180xc2x0-out-of-phase characteristics. The amplitude characteristic and the phase characteristic are referred to as xe2x80x9camplitude balancexe2x80x9d and xe2x80x9cphase balancexe2x80x9d between the balanced signal terminals, respectively.
The amplitude balance and the phase balance are defined as follows. If the SAW filter having a balance-to-unbalance conversion function is a three-port device consisting of, for example, a first port corresponding to an unbalanced input terminal and second and third ports corresponding to balanced output terminals, the amplitude balance=[A], where A=|20log(S21)|xe2x88x92|20log(S31)|, and the phase balance=Bxe2x88x92180, where B=|∠S21xe2x88x92∠S31|. In the above equations, S21 denotes the transfer coefficient from the first port to the second port, S31 denotes the transfer coefficient from the first port to the third port, and | | indicates an absolute value.
In balance between the balanced signal terminals, ideally, the amplitude balance and the phase balance are 0 dB and 0 degree, respectively, in the pass band of a SAW filter.
However, the SAW filter in the conventional art shown in FIG. 19 has a problem in that balance between the balanced signal terminals is low. As one possible reason, the distance (indicated by a portion 109 in FIG. 19) between an electrode finger connected to the balanced signal terminal 106 and a signal electrode finger of the IDT 102 is different from the distance (indicated by a portion 110 in FIG. 19) between an electrode finger connected to the balanced signal terminal 107 and a signal electrode finger of the IDT 103 by 0.5 times the wavelength defined by the pitch between the electrode fingers.
This leads to additional problems in that the total capacitance of the electrode fingers connected to the balanced signal terminal 106 is different from the that of the electrode fingers connected to the balanced signal terminal 107, and that the conversion efficiency between an electrical signal and a surface acoustic wave is different between the balanced signal terminals 106 and 107, resulting in low balance. FIG. 20 shows that the balanced signal terminal 107 shown in FIG. 19 is connected to the ground, and FIG. 21 shows that the balanced signal terminal 106 shown in FIG. 19 is connected to the ground. FIG. 22 shows the difference between the frequency-amplitude characteristic output from the balanced signal terminal 106 in FIG. 20 and the frequency-amplitude characteristic output from the balanced signal terminal 107 in FIG. 21. There is a large difference between the two amplitude characteristics, and this difference causes low balance.
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave filter having a balance-to-unbalance conversion function that compensates for the above-described difference between balanced signal terminals, which causes low balance between the balanced signal terminals, and that achieves satisfactory balance between the balanced signal terminals, and also the preferred embodiments of the present invention provide a communication apparatus including such a SAW filter.
According to a preferred embodiment of the present invention, a SAW filter includes a piezoelectric substrate, an IDT set disposed on the piezoelectric substrate, and a balanced signal terminal connected to the interdigital transducer set. The IDT set includes, along a SAW propagation direction, a plurality of IDTs for converting an input electrical signal into a surface acoustic wave and outputting the surface acoustic wave, and for converting a propagating surface acoustic wave into an output electrical signal and outputting the output electrical signal. The IDT set is asymmetric with respect to an imaginary axis which is located at the center of the IDT set in the SAW propagation direction and which is substantially perpendicular to the SAW propagation direction.
The IDT set may include an odd number of IDTs. The imaginary axis may be located at the approximately central portion of the IDT which is centrally located among the plurality of the IDTs, and may be substantially perpendicular to the SAW propagation direction.
The distances between adjacent IDTs in the IDT set are different such that the interdigital transducer set is asymmetric with respect to the imaginary axis.
The ratios (duties) of the electrode finger widths relative to the pitches of the IDT set may be different such that the interdigital transducer set is asymmetric with respect to the imaginary axis.
The pitches of the interdigital transducer set are different such that the interdigital transducer set is asymmetric with respect to the imaginary axis.
The IDT set may include narrower-pitch electrode finger portions at portions where two IDTs are adjacent to each other, each narrower-pitch electrode finger portion including several electrode fingers having a smaller pitch than that of the other electrode fingers. The pitches of the narrower-pitch electrode finger portions may be different such that the interdigital transducer set is asymmetric with respect to the imaginary axis.
With this configuration, the IDT set having a plurality of IDTs in the SAW propagation direction can achieve a filtering function which allows specific frequency band components to pass while other components to be stopped.
In this configuration, a balanced signal terminal connected to the IDT set does not have an electrically neutral point and therefore is of a floating balanced type, thus implementing a balance-to-unbalance conversion function while making it possible to relatively readily set an imaginary axis that is positioned at the approximate center of the IDT set in the SAW propagation direction and which is substantially perpendicular to the propagation direction.
In this configuration, the IDT set is configured so as to be asymmetric with respect to the imaginary axis. For example, the distances between adjacent IDTs are asymmetric with respect to the imaginary axis, the duties of the IDT set are asymmetric with respect to the imaginary axis, or the narrower-pitch electrode finger portions provided on the IDT set are asymmetric with respect to the imaginary axis. This increases balance, in particular, the phase balance.
Accordingly, a SAW filter performing both a filtering function and a balance-to-unbalance conversion function in which balance, in particular, the phase balance, is improved can be achieved, and can also be suitably used for communication apparatuses such as cellular telephones and other apparatuses.
In the SAW filter, preferably, at least one SAW resonator is connected at least in series or in parallel to the IDT set. The SAW resonator connected to the IDT set increases the filtering function.
In another preferred embodiment of the present invention, a communication apparatus includes a surface acoustic wave filter having any of the foregoing configurations according to various preferred embodiments described above. With use of a surface acoustic wave filter performing both a filtering function and a balance-to-unbalance conversion function in which balance, in particular, the phase balance, is improved, the communication capability is greatly improved.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description thereof with reference to the attached drawings.